What does 'Blender's System supports only quads' mean?
First let's solve some term confusion here. Every face of a mesh is a polygon. A tri is a polygon with three edges. A quad is a polygon with four edges. And a N-gon is a polygon with n edges. n stands as a variable here for any possible value. In 3D a N-gon describes a face with more than four edges. So when somebody talks about a N-gon then you know: he talks about a face / polygon with more than four edges.

Now for your question: The statement that Blender supports just tris or quads is not longer true. Since Blender can deal with N-Gons too since a pretty while. The code part that made this possible is called Bmesh. Which is a special code part under the hood of Blender to deal with the mesh geometry. It gots introduced with Blender 2.63. More to read here: https://wiki.blender.org/index.php/Dev:Ref/Release_Notes/2.63/BMesh